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Why you should diff your MongoDB

There are a number of situations where being able to conveniently analyze the differences between two database installations is extremely handy.

For example, you have a problem with your production system. All you know about it is that the problem lies somewhere in the database, but WHERE in the database? It worked fine yesterday, so if you compare the current database with yesterday’s backup, most likely you’ll find out what the problem is.

Additionally, it could be time to release an upgrade to the production system, but there are new configuration parameters required. The upgrade can be performed with a calm confidence when you can easily compare against the staging server and clearly see which parameters need to be introduced, changed or removed.

Or worse: an accidental update has removed vital configuration from the system, and it’s now crashing. You knew you should have taken a backup, but there just wasn’t time. And besides it was just a trivial change, so what could possibly go wrong?

But now, all you can do is sit and stare at your screen in horror and gulp down the bitter taste of regret. Unless of course, you can compare the differences and quickly identify which settings have been lost, and carry out an informed repair before anyone realizes your terrible blunder! (Of course, this NEVER HAPPENS on production systems, right? 🙂 )

More generally, being able to see how your data is changing is very powerful. Not only does it let you gain insight to your data, but it also provides confident control over the changes when you can see them explicitly and edit them directly.

Choose collections to compare

Simply drag the source database on top of the target database, and voilà! All collections with the same name in the source and target databases are automatically selected for comparison (highlighted in yellow in the screenshot below).

Of course, it’s not mandatory that collections need to have the same name. For example, if you need to compare the source collection current_customers with the target collection existing_customers, simply drag current_customers on top of existing_customers and it will be added to the comparison (highlighted in purple).

If you want to remove a particular pair of collections from the selection, simply right click it and choose Remove from comparison list. Alternatively, click the Remove button on the right of the comparison table.

Configure your data comparison

Studio 3T’s Data Compare and Sync feature also lets you configure data comparison settings, which can come in handy in case you only want to compare certain fields, filter based on certain criteria, etc.

To start, click on Configure to the right of the table. This will open up the Comparison Configuration window which has four tabs: Match On, Compare, Filter, and Projection. You can read about them in full here.

Run the analysis

OK, now that we’ve selected what we want to compare, hit the Run comparison button right above the table.

When the run is completed, a tab will open for each pair of collections. Let’s take a closer look at accounts.

In the image above, we can see the comparison took 792 milliseconds and found seven differences in total: five differing documents, one document that only appeared on the source side, and one document that only appeared on the target side.

The Overview tab shows exactly what was compared. This is useful if we save the results to come back to them later, by which time the comparison configuration may have been updated. The overview page of the results provides a permanent record of what was actually compared.

Spot the difference(s)

OK, now it’s time for the main course! Click on the Differences tab.

Differing documents are highlighted in yellow, source-only documents in green and target-only in red, though in nice pastel shades that are easy on the eye.

You can even just show the differing documents only for a cleaner, more targeted overview by clicking on View Options:

Now let’s expand a document.

Here we can see the same highlighting pattern: yellow shows fields that are different, green shows source-only fields, and red shows target-only fields.

You can expand or collapse all documents in one right-click.

Export comparison results

For a more convenient data analysis and easier sharing across teams, simply click on the Export button under the Differences tab to export results as a CSV file.

Sync MongoDB documents

We realize our source-only document should indeed be on the target collection, so we right click Copy Document to Target and confirm the copy operation.

Alright. Now we realize the target-only document is old and should no longer be present on the target.

No problem: We simply right-click and select Delete Document.

Sync MongoDB fields

OK, we’ve done some syncing at document level, now we sync individual fields. View Options > Show Differing Fields Only will really come in handy here.

Looking at the first document, it looks like ‘pet:manatee’ on the target side is incorrect, but instead should be ‘pet:cat’ as it is on the source side.

Right click on either ‘pet:manatee’ or ‘pet:cat’. Since the field exists on both source and target, you can choose to either ‘Copy Field from Source’ or ‘Copy Field to Target’, respectively.

You might also come across missing fields in the source document that should be copied over from the target document.

In this case, you would simply right click and select ‘Copy Field to Source’ if you clicked on the target side, or ‘Copy Field from Target’ if you clicked on the source side.

Studio 3T’s Data Compare and Sync lets you do a bi-directional sync on the same document, in that some changes are copied from the target document to the source, and other changes from the source to the target. Pretty neat, eh? 🙂

Sync MongoDB data via in-place editing

Alright, we’re done with the veg and potatoes, let’s center in on the steak!

For small changes, you don’t even need to invoke the right-click menu. Simply double-click a field and edit it directly – the diff status is updated in real time.

It’s been a pleasure

And that concludes our journey through how to compare MongoDB collections! I hope this tutorial on how to diff MongoDB has left you a little bit more powerful than when our journey first began.

Complement your new data comparison skills by checking out Schema Explorer, another nifty Studio 3T feature that will help tremendously in your data analysis. And if you haven’t downloaded our MongoDB GUI yet, here’s another chance 🙂

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About The Author

Graham Thomson

It all started with a Dragon 32… and has led to here, the most exciting place to be in software and databases right now. After helping hunt for the Higgs at CERN, and building large-scale, intelligent systems in industry, Graham is now doing the best job of all – building world-class tools that empower you to achieve world-class performance. Graham holds a Ph.D. in Computer Science from the University of Strathclyde.